Use None as default instead of dictionary

Co-authored-by: Hugo van Kemenade <[email protected]>

Tests/helper.py

@@ -256,7 +256,12 @@ def tostring(im: Image.Image, string_format: str, **options: dict[str, Any]) ->
     return out.getvalue()
 
 
-def hopper(mode: str | None = None, cache: dict[str, Image.Image] = {}) -> Image.Image:
+def hopper(
+    mode: str | None = None, cache: dict[str, Image.Image] | None = None
+) -> Image.Image:
+    if cache is None:
+        cache = {}
+
     if mode is None:
         # Always return fresh not-yet-loaded version of image.
         # Operations on not-yet-loaded images is separate class of errors